The present invention relates to a drawing head for ribbon optical cables, having optical fibre ribbons previously provided with end connectors.
In optical telecommunication cables there are very often used optical fibres which are gathered into groups of parallel fibres having a common plastic coating so as to form optical fibre ribbons.
Several optical fibre ribbons are then superposed and housed in respective longitudinal grooves of a carrying element referred to as "slotted spacer or core". The longitudinal grooves have a helical development, the helix being open or closed.
Anially along the length of the cable there is provided a strength member consisting, for example, of a cord made of metal, fibre glass or the like, whereas at the outside of the slotted core there are several coating layers having specific protective functions.
Optical cables of the above type are usually referred to as "ribbon cables".
Ribbon cables can be supplied by the manufacturer cut at the end and designed to be directly connected, at the time of installation, to respective destination equipment or to another cable, or connectors can be applied thereto in situ, which connectors in turn are adapted to enable the connection to the equipment or other cable. Alternatively, the ribbon cables can be previously equipped with connectors directly mounted by the cable manufacturer or the user in a place different from that designed for the cable installation.
Cables previously provided with connectors are preferred for many reasons, in particular because the connector-applying operations can be carried out under controlled conditions and in appropriate places which ensures the best results, and also because the cable laying operations are less expensive since the cable, if previously equipped with the connectors, is immediately ready for the intended connections.
However, while cables having bare ended fibre ribbons enable the end portion thereof to be removed if, on the laying, it looks damaged, ribbon cables already provided with connectors must be used as such and therefore damage to the connectors during the laying operations is not permissible.
It is known that cable laying involves the introduction of the cables into underground passages or ducts where, starting from one end, the cable is pulled over the whole length thereof.
This causes the cable and tape connectors to be unavoidably submitted to stresses of various kinds so that protection is particularly needed for the tapes.
In order to carry out the cable laying operations equipment referred to as "drawing heads" are known which are fastened to the pulling-resistant parts of the cable and provide grasping means, for example, in the form of eyebolts or the like, through which the cable can be seized and drawn into its housing.
In the case of cables provided with end connectors the drawing head must also provide an enclosure for the connectors capable of ensuring protection for the connectors as well.
In addition a drawing head must also accommodate an excess portion of fibre enabling the fibres to be connected at the intended positions after the cable support parts have been mechanically fastened to a rigid member which is a part of the destination equipment.
To this end, in known drawing heads, the excess portions of fibre ribbons are arranged to be kept in the form of skeins which are then unwound on the connection, respective rigid supports also being provided in the heads for receiving the connectors.
However, while these solutions can be usually accepted for high-capacity cables containing more than 400 fibres, for example, which are accommodated in large sized raceways, they are not convenient for low-capacity cables containing 100 fibres, for exampe, for which raceways of reduced diameter are provided.
In fact, when the excess portion of the optical fibre ribbons forming the ribbon cable are to be wound in the form of skeins, they are submitted to bending. It has been found that when small bending diameters, less than 30 mm, for instance, are kept for some time, for example when a cable equipped with the respective drawing head stays in stock for a given period, there is an unacceptable reduction in the transmission capabilities of the fibres.
Therefore, whereas cables of large diameter can accept drawing heads having a correspondingly large diameter, adapted to acccommodate excess portions of fibre ribbons in the form of skeins involving bending to a radius that does not induce significant attenuation in the fibre, drawing heads of the above diameters could not be accepted for cables of small diameter to be inserted in raceways, housings or the like with reduced clearance.
The availability of a drawing head for ribbon type optical cables having a reduced diameter but without causing too small bending radii for the fibres is, therefore, needed in order to provide a head adapted for all sizes of ribbon type optical cables.